This invention relates to an illumination optical system and, more particularly, to an illumination optical system suitably usable in an illumination optical device of a projection exposure apparatus for the manufacture of semiconductor devices, for example.
In projection exposure apparatuses for the manufacture of semiconductor devices, a circuit pattern formed on a reticle (mask) is projected onto a semiconductor substrate such as a wafer through a projection optical system, by which projection exposure is performed.
In conventional projection exposure apparatuses, light from a light source having a predetermined luminance distribution or from an image of such a light source is projected on a fly""s eye lens. With secondary light sources as defined in the neighborhood of the light exit surface of the fly""s eye lens, a condenser optical system illuminates a reticle having a circuit pattern formed thereon. In this manner, the surface to be illuminated (such as a reticle or a wafer) is illuminated uniformly.
It is known that, in the projection through a condenser optical system, as a condition for providing a uniform numerical aperture of an illumination optical system upon the surface to be illuminated, the fly""s eye lens should satisfy the sine condition. If the focal length of small lens elements constituting the fly""s eye lens is F, the largest incidence height of the illumination optical system parallel to the optical axis of the small lens elements is h, and the angle defined, with respect to the optical axis, by an emitted light ray having been incident at the largest incidence height is xcex8, then the sine condition can be expressed by:
h=Fxc2x7sin xcex8.
Generally, a fly""s eye lens comprises small lens elements of a rod-like shape having convex spherical surfaces at its opposite ends, being disposed two-dimensionally. With such a fly""s eye lens, the sine condition as above cannot be satisfied sufficiently and, therefore, there is a tendency that due to insufficient satisfaction of the sine condition the illuminance decreases from the center of the surface being illuminated toward its periphery.
Further, an anti-reflection film used in a lens has a characteristic that it reflects lights more with a larger incidence angle of light. Since generally light rays passing through the periphery of a lens have a larger incidence angle, also there is a tendency that the illuminance upon the surface being illuminated decreases from the center toward the periphery.
In consideration of this, in conventional projection exposure apparatuses, in order to prevent non-uniformness of illuminance wherein the illuminance at the periphery of the surface being illuminated is lower than that at the center thereof and thereby to attain uniform illuminance upon the surface being illuminated, a condenser optical system is used to positively produce distortion aberration such as shown in FIG. 15A, to thereby prevent a decrease in quantity of light impinging on the peripheral portion.
When a condenser optical system effective to produce distortion aberration of FIG. 15A is used, due to an unchangeable quantity of Helmholz-Lagrange, the secondary light sources as can be observed from various points on the surface being illuminated differ in size, between points on the axis and off the axis, such as shown in FIG. 15B. For example, in FIG. 15B, the secondary light sources as can be observed at different points have the same shape, at any point within the picture field. However, if the largest light intensity is 100 and comparing diameters of contour lines of light intensity 20, it is seen that the diameter is larger at points off the axis, as compared with the diameter at a point on the axis. Namely, in conventional projection exposure apparatuses, a condenser lens is used to positively produce distortion aberration and, as a result of this, secondary light sources observed on the axis and off the axis, upon the surface being illuminated, differ in size ("sgr" value).
In recent projection exposure apparatuses for the manufacture of semiconductor devices, enlargement of integration of VLSI has required a higher image uniformness over the whole surface being illuminated, than required before. In order to meet this, the illumination optical system should provide uniformness of illuminance upon the surface to be illuminated as well as uniformness, over the whole surface to be illuminated, in size of secondary light sources as can be observed from various points on the surface being illuminated.
Here, if the numerical aperture of an illumination optical system is NAil and the numerical aperture of a projection optical system is NApo, the secondary light sources (effective light sources) can be expressed by a value:
"sgr"=NAil/NApo
For example, in accordance with results of experiments made to a projection exposure apparatus under the condition that the wavelength is 248 nm and the image side numerical aperture NA is 0.6, it has been found that, when "sgr" changes by 0.1, the linewidth of an isolated line of 0.3 micron linewidth changes by about 20 nm. From the uniformness of linewidth within the picture field as currently being required, the effect of a secondary light source ("sgr") difference in the surface to be illuminated, upon the linewidth, should be kept low of about 5 nm or less. In order to attain this, if the largest diameter of those of the secondary light sources observed at various points on the surface being illuminated, which can be regarded as having the same intensity ratio, is denoted by "sgr"max while the smallest diameter thereof is denoted by "sgr"min, preferably the balance difference of "sgr"max and "sgr"min should be not greater than 3%, at the maximum.
However, with illumination optical systems conventionally used in projection exposure apparatuses, it is difficult to satisfy both the uniformness of illuminance and the uniformness of illumination numerical aperture, upon the surface being illuminated.
It is an object of the present invention to provide an illumination optical system and/or a projection exposure apparatus by which both the uniformness of illuminance and the uniformness of illumination numerical aperture, upon the surface being illuminated, can be improved.
In accordance with a first aspect of the present invention, there is provided an illumination optical system, comprising: secondary light source forming means for forming secondary light sources by use of light from a light source; and optical means for illuminating a surface to be illuminated, with light from the secondary light sources; wherein the secondary light sources as seen from points on the surface illuminated have substantially the same numerical aperture and wherein illuminance on the surface illuminated is substantially uniform.
In accordance with a second aspect of the present invention, there is provided an illumination optical system, comprising: secondary light source forming means for forming secondary light sources by use of light from a light source; optical means for illuminating a surface to be illuminated, with light from the secondary light sources, wherein said optical means is arranged to assure that the secondary light sources as seen from points on the surface illuminated have substantially the same numerical aperture; and illuminance uniforming means effective to make the illuminance on the surface illuminated uniform.
In accordance with a third aspect of the present invention, there is provided an illumination optical system, comprising: a fly""s eye lens for forming secondary light sources by use of light from a light source, and optical means for illuminating a surface to be illuminated, with light from the secondary light sources, wherein said fly""s eye lens has a plurality of small lens elements each comprising an aspherical surface lens.
In accordance with a fourth aspect of the present invention, there is provided an illumination optical system, comprising: a fly""s eye lens for forming secondary light sources by use of light from a light source; optical means for illuminating a surface to be illuminated, with light from the secondary light sources, wherein said fly""s eye lens has a plurality of small lens elements each comprising a diffractive optical element.
In accordance with a fifth aspect of the present invention, there is provided an illumination optical system, comprising: secondary light source forming means for forming secondary light sources by use of light from a light source; optical means for illuminating a surface to be illuminated, with light from the secondary light sources; scanning means for scanning the surface to be illuminated, wherein the surface to be illuminated is illuminated while being scanned by said scanning means; wherein said optical means is arranged to assure that the secondary light sources as seen from points on the surface illuminated have substantially the same numerical aperture; and a variable slit disposed at or adjacent to a plane optically conjugate with the surface to be illuminated, for changing a width in a scan direction of an illumination region upon the surface to be illuminated, such that the width in the scan direction of the illumination region can be changed by said variable slit in accordance with an illuminance distribution on the illumination region.
In accordance with a sixth aspect of the present invention, there is provided an illumination optical system, comprising: secondary light source forming means for forming secondary light sources by use of light from a light source; a stop for restricting the shape of the secondary light sources; optical means for illuminating a surface to be illuminated, with light from the secondary light sources; scanning means for scanning the surface to be illuminated, wherein the surface to be illuminated is illuminated while being scanned by said scanning means; and a variable slit disposed at or adjacent to a plane optically conjugate with the surface to be illuminated, for changing a width in a scan direction of an illumination region upon the surface to be illuminated, such that the width in the scan direction of the illumination region can be changed by said variable slit in accordance with a change in shape of the secondary light sources.
In accordance with a seventh aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system including (i) secondary light source forming means for forming secondary light sources by use of light from a light source, (ii) optical means for directing light from the secondary light sources to a reticle; a projection optical system for projecting a pattern, formed on the reticle, onto the surface of a wafer; wherein said optical means is arranged to assure that the secondary light sources as seen from points on a surface being illuminated have substantially the same numerical aperture; and illuminance uniforming means effective to make the illuminance on the surface illuminated uniform.
In accordance with an eighth aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system including (i) a fly""s eye lens for forming secondary light sources by use of light from a light source, and (ii) optical means for directing light from the secondary light sources to a reticle; and a projection optical system for projecting a pattern formed on the reticle, onto a surface of a wafer, wherein said fly""s eye lens has a plurality of small lens elements each comprising an aspherical surface lens.
In accordance with a ninth aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system including (i) a fly""s eye lens for forming secondary light sources by use of light from a light source, and (ii) optical means for directing light from the secondary light sources to a reticle; and a projection optical system for projecting a pattern formed on the reticle, onto a surface of a wafer; wherein said fly""s eye lens has a plurality of small lens elements each comprising a diffractive optical element.
In accordance with a tenth aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system including (i) secondary light source forming means for forming secondary light sources by use of light from a light source, and (ii) optical means for directing light from the secondary light sources to a reticle; a projection optical system for projecting a pattern formed on the reticle, onto a surface of a wafer; scanning means for scanning the reticle and the wafer in synchronism with each other, wherein the pattern is transferred to the wafer with the scan by said scanning means; wherein said optical means is arranged to assure that the secondary light sources as seen from points on a surface illuminated have substantially the same numerical aperture; and a variable slit disposed at or adjacent to a plane optically conjugate with the surface to be illuminated, for changing a width in a scan direction of an illumination region upon the surface to be illuminated, such that the width in the scan direction of the illumination region can be changed by said variable slit in accordance with an illuminance distribution on the illumination region.
In accordance with an eleventh aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system including (i) secondary light source forming means for forming secondary light sources by use of light from a light source, and (ii) optical means for directing light from the secondary light sources to a reticle; a projection optical system for projecting a pattern of the reticle, onto a surface of a wafer; scanning means for scanning the reticle and the wafer in synchronism with each other, wherein the pattern is transferred to the wafer with the scan by said scanning means; wherein said optical means is arranged to assure that the secondary light sources as seen from points on a surface illuminated have substantially the same numerical aperture; and a variable slit disposed at or adjacent to a plane optically conjugate with the surface to be illuminated, for changing a width in a scan direction of an illumination region upon the surface to be illuminated, such that the width in the scan direction of the illumination region can be changed by said variable slit in accordance with a change in shape of the secondary light sources.
In accordance with a twelfth aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: transferring, by exposure, a pattern formed on a reticle onto a wafer with use of a projection exposure apparatus in accordance with any one of the seventh to eleventh aspects of the present invention as described above; and developing the pattern transferred to the wafer.
In accordance with a thirteenth aspect of the present invention, there is provided a fly""s eye lens, comprising: a plurality of small lens elements at least one of which has at least one aspherical lens surface.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.